Fluoride fluorescent material and method for producing the same as well as light emitting device using the same

ABSTRACT

The present invention provides a method for producing a fluoride fluorescent material, comprising: contacting a fluoride particles represented by the following general formula (I):
 
K 2 [M 1−a Mn 4+   a F 6 ]  (I)
 
wherein M is at least one member selected from the group consisting of elements belonging to Groups 4 and 14 of the Periodic Table, and a satisfies the relationship: 0&lt;a&lt;0.2; with a solution containing alkaline earth metal ions in the presence of a reducing agent to form an alkaline earth metal fluoride on the surface of the fluoride particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application based on U.S. patent application Ser.No. 14/469,816, filed Aug. 27, 2014, which claims priority under 35 USC119 from Japanese patent Application No. 2013-177685, filed on Aug. 29,2013, the disclosures of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a fluoride fluorescent material and amethod for producing the same as well as a light emitting device usingthe same.

Description of the Related Art

A light emitting diode (LED) is a light emitting device frequently usedas a substitute for a conventional light source, and is useful as adisplay lamp, a warning lamp, and a lamp for indicator or lighting. Likethe light emitting diode, with respect to a laser diode (LD), variouslight emitting devices using a laser diode and a fluorescent material incombination have been proposed. Various types of light emitting devicesemitting light of, e.g., white color, electric bulb color, or orangecolor have been developed, wherein the devices use a semiconductor lightemitting element produced from a Group III-V alloy, such as galliumnitride (GaN), and a fluorescent material in combination. These lightemitting devices emitting light of white color or the like is controlledin color tone by the principle of mixing colors of lights. As a systemfor emitting a white light, a system using a light emitting elementemitting an ultraviolet light and three types of fluorescent materialsrespectively emitting red (R), green (G), and blue (B) lights, and asystem using a light emitting element emitting a blue light and afluorescent material emitting, e.g., a yellow light have been wellknown. A light emitting device of a system using a light emittingelement emitting a blue light and a fluorescent material emitting, e.g.,a yellow light is demanded in a wide variety of fields, such as alighting, a car lighting, a display, and a backlight for liquid crystal.In these fields, the fluorescent material used in the displayapplication is desired to have both excellent light emission efficiencyand excellent intensity of color for reproducing colors in a wide rangeon the chromaticity coordinates. Further, the fluorescent material usedin the display application is desired to be advantageously used incombination with a filter, and thus a fluorescent material exhibiting anemission spectrum having a narrow half band width is demanded.

For example, as a red light-emitting fluorescent material having anexcitation band in the blue region, and exhibiting an emission spectrumhaving a narrow half band width, a fluoride fluorescent material havinga composition, such as K₂TiF₆:Mn⁴⁺, Ba₂TiF₆:Mn⁴⁺, Na₂TiF₆:Mn⁴⁺, orK₃ZrF₇:Mn⁴⁺, has been known (see, for example, Japanese PatentApplication prior-to-examination Publication (kohyo) No. 2009-528429).In addition, a fluoride fluorescent material having a composition:K₂SiF₆:Mn⁴⁺ has been known (see, for example, Japanese Unexamined PatentPublication No. 2010-209311). Further, the excitation and emissionspectra and the light emission mechanism of a fluoride complexfluorescent material activated by Mn⁴⁺have been known (see, for example,“Effective Mn(IV) Emission in Fluoride Coordination”, written by A. G.Paulusz, J. Electrochemical Soc., 120 N7, 1973, p. 942-947).

SUMMARY OF THE INVENTION

A method for producing a fluoride fluorescent material, the methodcomprising:

contacting fluoride particles represented by the following generalformula (I):K₂[M_(1−a)Mn⁴⁺ _(a)F₆]  (I)wherein M is at least one member selected from the group consisting ofelements belonging to Groups 4 and 14 of the Periodic Table, and asatisfies the relationship: 0<a<0.2; with a solution containing alkalineearth metal ions in the presence of a reducing agent to form an alkalineearth metal fluoride on the surface of the fluoride particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view diagrammatically showing the fluoride fluorescentmaterial of the present embodiment.

FIG. 2 is a view diagrammatically showing the fluoride fluorescentmaterial of the present embodiment.

FIG. 3 is a diagrammatic cross-sectional view showing the light emittingdevice of the present embodiment.

FIG. 4 is a diagrammatic plan view showing the light emitting device ofthe present embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The above-mentioned red light-emitting fluoride fluorescent materialsactivated by tetravalent manganese ions exhibiting an emission spectrumhaving a narrow half band width and light emitting devices using themare considered advantageous particularly to the application of backlightfor liquid crystal, and desired to be brought into practical use.However, the conventional fluoride fluorescent materials activated bytetravalent manganese ions are not satisfactory in water resistance andthus have a problem about the long-term reliability.

In view of the above-mentioned problems accompanying the conventionaltechniques, the present invention has been made. It is a primary objectof the present invention to provide a red light-emitting fluorescentmaterial having excellent water resistance and a method for producingthe same as well as a light emitting device using the same.

The present inventors have conducted extensive and intensive studieswith a view toward solving the above-mentioned problems. As a result,the present invention has been completed. The present invention involvesthe following embodiments.

An embodiment is a method for producing a fluoride fluorescent material,wherein the method comprising: contacting fluoride particles representedby the following general formula (I):K₂[M_(1−a)Mn⁴⁺ _(a)F₆]  (I)wherein M is at least one member selected from the group consisting ofelements belonging to Groups 4 and 14 of the Periodic Table, and asatisfies the relationship: 0<a<0.2; with a solution containing alkalineearth metal ions in the presence of a reducing agent to form an alkalineearth metal fluoride on the surface of the fluoride particles.

In other words, the embodiment is a method for producing a fluoridefluorescent material, wherein the method comprising the step ofcontacting fluoride particles represented by the following generalformula (I):K₂[M_(1−a)Mn⁴⁺ _(a)F₆]  (I)wherein M is at least one member selected from the group consisting ofelements belonging to Groups 4 and 14 of the Periodic Table, and asatisfies the relationship: 0<a<0.2; with a solution containing alkalineearth metal ions in the presence of a reducing agent to form an alkalineearth metal fluoride on the surface of the fluoride particles.

Another embodiment of the present invention is a fluoride fluorescentmaterial comprising: fluoride particles represented by the followinggeneral formula (I):K₂[M_(1−a)Mn⁴⁺ _(a)F₆]  (I)wherein M is at least one element selected from the group consisting ofelements belonging to Groups 4 and 14 of the Periodic Table, and asatisfies the relationship: 0<a<0.2; and an alkaline earth metalfluoride on the surface of the fluoride particles.

Still another embodiment of the present invention is a light emittingdevice comprising: a light source that generates light in the wavelengthrange of from 380 to 485 nm; and the fluoride fluorescent materialproduced by the above-mentioned method for producing a fluoridefluorescent material or the above-mentioned fluoride fluorescentmaterial.

Still further another embodiment of the present invention is an imagedisplay apparatus comprising the above-mentioned light emitting device.

In the present invention, there can be provided a red light-emittingfluorescent material having excellent water resistance and a method forproducing the same as well as a light emitting device using the same.

The embodiment described below exemplifies the fluoride fluorescentmaterial, method for producing the same, and light emitting device forspecifically showing the technical idea of the present invention, andthe present embodiment is not limited to the following fluoridefluorescent material, method for producing the same, and light emittingdevice.

The relationship between the color names and the chromaticitycoordinates, the relationship between the wavelength ranges of light andthe color names of monochromatic light, and others are according to JISZ8110. Specifically, a wavelength range of 380 to 455 nm corresponds tobluish violet, 455 to 485 nm corresponds to blue, 485 to 495 nmcorresponds to bluish green, 495 to 548 nm corresponds to green, 548 to573 nm corresponds to yellowish green, 573 to 584 nm corresponds toyellow, 584 to 610 nm corresponds to yellowish red, and 610 to 780 nmcorresponds to red.

In the present specification, the term “step” means not only anindependent step but also a step which cannot be clearly distinguishedfrom the other steps but can achieve the desired object thereof. Therange of values expressed using “to” indicates a range which includesthe figures shown before and after “to” as, respectively, the minimumvalue and the maximum value. Further, with respect to the amount of acomponent contained in the composition, when a plurality of materialsare present in the composition as the components of the composition, theamount of the components means the total amount of the materials presentin the composition unless otherwise specified.

<Method for Producing a Fluoride Fluorescent Material>

The method for producing a fluoride fluorescent material of the presentembodiment comprises: contacting fluoride particles represented by thegeneral formula (I) below with a solution containing alkaline earthmetal ions in the presence of a reducing agent to form an alkaline earthmetal fluoride on the surface of the fluoride particles. In the formula(I), M is at least one member selected from the group consisting ofelements belonging to Groups 4 and 14 of the Periodic Table, and asatisfies the relationship: 0<a<0.2.K₂[M_(1−a)Mn⁴⁺ _(a)F₆]  (I)

The fluoride fluorescent material produced by the method of the presentembodiment is a red light-emitting fluorescent material exhibiting anemission spectrum having a narrow half band width, and has excellentlight emission properties, and can exhibit satisfactory durability in along-term reliability test.

Generally, in the fluoride fluorescent material represented by thegeneral formula (I), it is considered that tetravalent manganese ionsconstituting the fluoride fluorescent material react with water in thesurface region of the particles to form manganese dioxide, so that thesurface of the particles is colored black, lowering the luminance. Forthis reason, the fluoride fluorescent material cannot achievesatisfactory durability in a long-term reliability test, and has aproblem in that it is difficult to apply the fluoride fluorescentmaterial to the use which requires high reliability.

When the fluoride fluorescent material is placed in an aqueous solutioncontaining alkaline earth metal ions, the fluoride particles suffer adissolution reaction, so that ions of the metal constituting thefluoride particles and fluorine ions are formed. In this instance, thefluorine ions react with the alkaline earth metal ions to form analkaline earth metal fluoride on the surface of the fluoride particles,making it possible to prevent the fluoride particles from furthersuffering a dissolution reaction. On the other hand, the tetravalentmanganese ions are reduced to bivalent manganese ions due to thereducing agent present in the solution, so that the formation ofmanganese dioxide is suppressed.

As mentioned above, in the fluoride fluorescent material produced by themethod of the present embodiment, an alkaline earth metal fluoride isformed on the surface of the fluoride fluorescent material, and furtherthe presence of the reducing agent suppresses the formation of manganesedioxide in the surface. For this reason, the fluoride fluorescentmaterial is considered to have high light emission intensity and to besuppressed in a lowering of the luminance for a long term. Therefore, itis considered that the fluoride fluorescent material can achieveexcellent long-term reliability.

In the present embodiment, in the fluoride fluorescent materialrepresented by the general formula (I), M is at least one memberselected from the group consisting of elements belonging to Groups 4 and14 of the Periodic Table. It is preferred that M is at least one memberselected from the group consisting of titanium (Ti), zirconium (Zr),hafnium (Hf), silicon (Si), germanium (Ge), and tin (Sn). It is furtherpreferred that M comprises silicon (Si), or silicon (Si) and germanium(Ge).

With respect to a, there is no particular limitation as long as asatisfies the relationship: 0<a<0.2, and a can be appropriately selectedaccording to, e.g., intended light emission properties. a can beadjusted to a desired range by, for example, controlling theconcentration of ions of the raw material compound in thebelow-mentioned method for producing fluoride particles.

The solution containing alkaline earth metal ions comprises at leastalkaline earth metal ions, counter ions, and water. Examples of alkalineearth metal ions include magnesium (Mg) ions, calcium (Ca) ions, andstrontium (Sr) ions.

Of these, from the viewpoint of achieving excellent emission luminanceand water resistance, it is preferred that the alkaline earth metal ionscomprise calcium ions.

The solution containing alkaline earth metal ions is obtained in theform of an aqueous solution of a compound comprising an alkaline earthmetal, and may contain another component (e.g., an alcohol, such asmethanol or ethanol) if necessary. Examples of compounds comprising analkaline earth metal include alkaline earth metal nitrates {e.g.,Mg(NO₃)₂, Ca(NO₃)₂, and Sr(NO₃)₂}, alkaline earth metal acetates {e.g.,Mg(CH₃CO₂)₂, Ca(CH₃CO₂)₂, and Sr(CH₃CO₂)₂}, alkaline earth metalchlorides {e.g., MgCl₂, CaCl₂, and SrCl₂}, alkaline earth metal iodides{e.g., MgI₂, Cal₂, and SrI₂}, and alkaline earth metal bromides {e.g.,MgBr₂, CaBr₂, and SrBr₂}.

With respect to the compound comprising an alkaline earth metal, asingle type may be used, or two or more types may be used incombination.

With respect to the concentration of the alkaline earth metal in thesolution containing alkaline earth metal ions, there is no particularlimitation. The lower limit of the alkaline earth metal concentration ofthe solution containing alkaline earth metal ions is, for example, 0.01%by weight or more, preferably 0.03% by weight or more, more preferably0.05% by weight or more. Further, the upper limit of the alkaline earthmetal concentration of the solution containing alkaline earth metal ionsis, for example, 5% by weight or less, preferably 3% by weight or less,more preferably 2% by weight or less.

In the present embodiment, the amount of the solution containingalkaline earth metal ions is, relative to 100 parts by weight of thefluoride particles, preferably 100 to 3,000 parts by weight, morepreferably 200 to 2,000 parts by weight. When the solution containingalkaline earth metal ions in such an amount is used, the waterresistance is further improved.

At least part of tetravalent manganese ions caused due to a reactionbetween the fluoride particles and the solution containing alkalineearth metal ions are reduced to bivalent manganese ions by the presenceof a reducing agent. Specifically, by adding a reducing agent, 90 mol %or more of tetravalent manganese ions caused due to a reaction betweenthe fluoride particles and the solution containing alkaline earth metalions are preferably reduced, more preferably, 95 mol % or more of thetetravalent manganese ions are reduced.

With respect to the reducing agent, there is no particular limitation aslong as it can reduce tetravalent manganese ions. Specific examples ofreducing agents include hydrogen peroxide and oxalic acid. Of these,preferred is hydrogen peroxide because hydrogen peroxide can reducemanganese without adversely affecting the base of the fluorideparticles, for example, without dissolving the fluoride particles, andhydrogen peroxide is finally decomposed into harmless water and oxygen,and hence can be easily used in the production process and causes lessload on the environment.

With respect to the amount of the reducing agent added, there is noparticular limitation. The amount of the reducing agent added can beappropriately selected according to, for example, the content of themanganese in the fluoride particles, but a preferred amount of thereducing agent added is such that the base of the fluoride particles isnot adversely affected. The amount of the reducing agent added is,specifically, preferably 1 equivalent % or more, more preferably 3equivalent % or more, based on the amount of the manganese contained inthe fluoride particles.

1 Equivalent means mole(s) of the reducing agent required to reduce 1mole of tetravalent manganese ions to bivalent manganese ions.

The lower limit of the reducing agent concentration of the solutioncontaining alkaline earth metal ions is, for example, 0.01% by weight ormore, preferably 0.03% by weight or more, more preferably 0.05% byweight or more. Further, the upper limit of the reducing agentconcentration of the solution containing alkaline earth metal ions is,for example, 5% by weight or less, preferably 3% by weight or less, morepreferably 2% by weight or less.

(Contacting Method)

With respect to the method for contacting the fluoride particles withthe solution containing alkaline earth metal ions in the presence of areducing agent, there is no particular limitation. As an example of thecontacting method, there can be mentioned a method in which the reducingagent, the fluoride particles, and the solution containing alkalineearth metal ions are mixed with each other.

(Contacting Time)

With respect to the time during which the fluoride particles and thesolution containing alkaline earth metal ions are contacted in thepresence of a reducing agent, there is no particular limitation as longas an alkaline earth metal fluoride can be formed on the surface of thefluoride particles. The contacting time can be, for example, 10 minutesto 10 hours, preferably 30 minutes to 5 hours.

(Reaction Temperature)

With respect to the temperature at which the reducing agent, thefluoride particles, and the solution containing alkaline earth metalions are mixed with each other, there is no particular limitation. Forexample, they can be mixed at a temperature in the range of from 15 to40° C., preferably at a temperature in the range of from 23 to 28° C.

Further, with respect to the atmosphere for the mixing, there is noparticular limitation. The mixing may be performed in general air or inan atmosphere of an inert gas, such as nitrogen gas.

[Other Steps]

The method for producing a fluoride fluorescent material may furthercomprise another step if necessary. For example, the fluoridefluorescent material obtained by the method of the present embodimentcan be recovered by solid-liquid separation, such as filtration. Theobtained fluoride fluorescent material may be washed with a solvent,such as ethanol, isopropyl alcohol, water, or acetone. Further, theobtained fluoride fluorescent material may be subjected to dryingtreatment. In such a case, the obtained fluoride fluorescent material isdried at, for example, 50° C. or higher, preferably 55° C. or higher,more preferably 60° C. or higher, and at, for example, 110° C. or lower,preferably 100° C. or lower, more preferably 90° C. or lower. Withrespect to the time for drying, there is no particular limitation aslong as the moisture attached to the fluoride fluorescent material canbe evaporated, and the drying time is, for example, about 10 hours.

In the fluorescent material particles in the present embodiment, thefluoride particles forming a fluorescent material and alkaline earthmetal ions are reacted to form an alkaline earth metal fluoride on thesurface of the fluoride particles. Thus, the probability of crystals,which are soluble in water, present on the surface of the fluorideparticles is reduced, so that even when the surface of the fluorideparticles is dissolved out in, e.g., moisture, the formation ofmanganese dioxide is suppressed to prevent the surface from beingcolored black, making it possible to suppress a lowering of the lightemission intensity. In the present embodiment, the alkaline earth metalfluoride is preferably formed in the form of a film on the surface ofthe fluoride particles. When the alkaline earth metal fluoride ispresent in the form of a film on the surface of the fluoride particles,while suppressing the dissolution of the surface in, e.g., moisture, thelight extraction efficiency can be improved. Thus, a fluorescentmaterial having high luminance and excellent water resistance isobtained.

Further, it is preferred that the alkaline earth metal fluoride iscalcium fluoride.

[Method for Producing Fluoride Particles]

The method of the present embodiment may further comprise the step ofobtaining fluoride particles.

In the present embodiment, the fluoride particles are obtained by, forexample, a method comprising the step of contacting, in a liquid mediumcomprising hydrogen fluoride, first complex ions comprising tetravalentmanganese ions, potassium ions, and second complex ions comprising atleast one member selected from the group consisting of elementsbelonging to Groups 4 and 14 of the Periodic Table with one another toobtain fluoride particles represented by the general formula (I).

With respect to the method for contacting the first complex ions,potassium ions, and the second complex ions with one another, there isno particular limitation. For example, there can be mentioned a methodin which a first solution containing the first complex ions, a secondsolution containing potassium ions, and a third solution containing thesecond complex ions are mixed with each other. In this case, hydrogenfluoride may be contained in any of the first to third solutions, or thefirst to third solutions may be mixed in a liquid medium comprisinghydrogen fluoride.

From the viewpoint of achieving excellent formation efficiency of thefluoride particles and excellent light emission properties, the methodfor producing the fluoride particles preferably comprises mixingtogether a first solution containing the first complex ions, a secondsolution containing potassium ions, and a third solution containing thesecond complex ions, more preferably comprises mixing together a firstsolution containing the first complex ions and hydrogen fluoride, asecond solution containing potassium ions and hydrogen fluoride, and athird solution containing the second complex ions.

(First Solution)

The first solution (hereinafter, also referred to as “solution A”)contains at least the first complex ions comprising tetravalentmanganese ions, and may contain another component if necessary. It ispreferred that the first solution further contains hydrogen fluoride inaddition to the first complex ions.

The first solution is obtained in the form of, for example, an aqueoussolution of hydrofluoric acid containing a tetravalent manganese ionsource. With respect to the manganese source, there is no particularlimitation as long as it is a compound containing tetravalent manganeseions. Specific examples of manganese sources which can constitute thefirst solution include K₂MnF₆, KMnO₄, and K₂MnCl₆. Of these, preferredis K₂MnF₆ because, for example, K₂MnF₆ does not contain chlorine whichis likely to strain the crystal lattice to cause the crystal to beunstable, and K₂MnF₆ can be stably present as MnF₆ complex ions inhydrofluoric acid while maintaining the oxidation number (tetravalent)which can achieve activation. Among the manganese sources, onecontaining potassium can serve also as a potassium source contained inthe second solution.

With respect to the manganese source constituting the first solution, asingle type may be used, or two or more types may be used incombination.

With respect to the concentration of the first complex ions in the firstsolution, there is no particular limitation. The lower limit of thefirst complex ion concentration of the first solution is, for example,0.1% by weight or more, preferably 0.3% by weight or more, morepreferably 0.5% by weight or more. Further, the upper limit of the firstcomplex ion concentration of the first solution is, for example, 5% byweight or less, preferably 3% by weight or less, more preferably 2% byweight or less. The first complex ion concentration can be determined bymaking a calculation from the amounts of the components charged uponpreparing the first solution. The concentration of each component in thesolution shown below is similarly determined.

When the first solution contains hydrogen fluoride, the lower limit ofthe hydrogen fluoride concentration of the first solution is, forexample, 30% by weight or more, preferably 35% by weight or more, morepreferably 40% by weight or more. Further, the upper limit of thehydrogen fluoride concentration of the first solution is, for example,70% by weight or less, preferably 65% by weight or less, more preferably60% by weight or less. When the hydrogen fluoride concentration is 30%by weight or more, the manganese source (for example, K₂MnF₆)constituting the first solution is improved in the stability tohydrolysis, so that a change of the tetravalent manganese ionconcentration of the first solution is suppressed. Thus, it is likelythat the amount of the manganese for activation contained in theobtained fluoride particles can be easily controlled, making it possibleto suppress variation (change) of the light emission efficiency of thefluoride particles. On the other hand, when the hydrogen fluorideconcentration is 70% by weight or less, a lowering of the boiling pointof the first solution is suppressed, so that the generation of hydrogenfluoride gas is suppressed. Thus, the hydrogen fluoride concentration ofthe first solution can be easily controlled, making it possible toeffectively suppress variation (change) of the particle diameter of theobtained fluoride particles.

The first solution may, in addition to the first complex ions, furthercontain the second complex ions comprising at least one member selectedfrom the group consisting of elements belonging to Groups 4 and 14 ofthe Periodic Table. The second complex ions and preferred modes thereofare described in detail below.

When the first solution contains the second complex ions in addition tothe first complex ions, the lower limit of the second complex ionconcentration of the first solution is not particularly limited, and maybe, for example, 1% by weight or more. The upper limit of the secondcomplex ion concentration of the first solution is, for example, 30% byweight or less, preferably 25% by weight or less, more preferably 20% byweight or less.

(Second Solution)

The second solution (hereinafter, also referred to as “solution B”)contains at least potassium ions, and may contain another component ifnecessary. It is preferred that the second solution further containshydrogen fluoride in addition to potassium ions.

The second solution is obtained in the form of, for example, an aqueoussolution of hydrofluoric acid containing potassium ions. Specificexamples of potassium sources containing potassium ions which canconstitute the second solution include water-soluble potassium salts,such as KF, KHF₂, KOH, KCl, KBr, Kl, CH₃CO₂K, and K₂CO₃. Of these,preferred is KHF₂ because KHF₂ can be dissolved without lowering thehydrogen fluoride concentration of the solution, and further has smallheat of dissolution and hence achieves high safety.

With respect to the potassium source constituting the second solution, asingle type may be used, or two or more types may be used incombination.

The lower limit of the potassium ion concentration of the secondsolution is, for example, 10% by weight or more, preferably 12.5% byweight or more, more preferably 15% by weight or more. Further, theupper limit of the potassium ion concentration of the second solutionis, for example, 35% by weight or less, preferably 32.5% by weight orless, more preferably 30% by weight or less. When the potassium ionconcentration is 10% by weight or more, it is likely that the yield ofthe fluoride fluorescent material is improved. On the other hand, whenthe potassium ion concentration is 35% by weight or less, it is likelythat the obtained fluoride particles have an increased particlediameter.

When the second solution contains hydrogen fluoride, the lower limit ofthe hydrogen fluoride concentration of the second solution is, forexample, 30% by weight or more, preferably 35% by weight or more, morepreferably 40% by weight or more. Further, the upper limit of thehydrogen fluoride concentration of the second solution is, for example,70% by weight or less, preferably 65% by weight or less, more preferably60% by weight or less.

(Third Solution)

The third solution (hereinafter, also referred to as “solution C”)contains at least the second complex ions comprising at least one memberselected from the group consisting of elements belonging to Groups 4 and14 of the Periodic Table, and may contain another component ifnecessary. It is preferred that the second complex ions further comprisefluorine ions in addition to the at least one member selected from thegroup consisting of elements belonging to Groups 4 and 14 of thePeriodic Table.

The third solution is obtained in the form of, for example, an aqueoussolution containing the second complex ions.

The second complex ions preferably comprise at least one member selectedfrom the group consisting of titanium (Ti), zirconium (Zr), hafnium(Hf), silicon (Si), germanium (Ge), and tin (Sn), more preferablycomprise silicon (Si), or silicon (Si) and germanium (Ge), furtherpreferably are silicon fluoride complex ions.

For example, when the second complex ions comprise silicon (Si), thesecond complex ion source is preferably a compound containing siliconand fluoride ions and having excellent solubility in the solution.Specific examples of second complex ion sources include H₂SiF₆, Na₂SiF₆,(NH₄)₂SiF₆, Rb₂SiF₆, and Cs₂SiF₆. Of these, H₂SiF₆ is preferred becauseH₂SiF₆ has high solubility in water and contains no alkali metal elementas an impurity.

With respect to the second complex ion source constituting the thirdsolution, a single type may be used, or two or more types may be used incombination.

The lower limit of the second complex ion concentration of the thirdsolution is, for example, 10% by weight or more, preferably 15% byweight or more, more preferably 20% by weight or more. Further, theupper limit of the second complex ion concentration of the thirdsolution is, for example, 60% by weight or less, preferably 55% byweight or less, more preferably 50% by weight or less.

With respect to the method for mixing the first solution, secondsolution, and third solution, there is no particular limitation, and,while stirring the first solution, the second solution and thirdsolution may be added to the first solution, while stirring the thirdsolution, the first solution and second solution may be added to thethird solution, or, while stirring the second solution, the firstsolution and third solution may be added to the second solution.Alternatively, the first solution, second solution, and third solutionmay be individually placed in a vessel and mixed with each other bystirring.

Of these, from the viewpoint of efficiently obtaining the fluorideparticles, the method for mixing the first solution, second solution,and third solution is preferably a method in which, while stirring thefirst solution, the second solution and third solution are added to thefirst solution.

Mixing means for mixing the first solution, second solution, and thirdsolution can be appropriately selected from mixing means generally usedaccording to, e.g., the reaction vessel.

With respect to the temperature at which the first solution, secondsolution, and third solution are mixed together, there is no particularlimitation. For example, the solutions can be mixed together at atemperature in the range of from 15 to 40° C., preferably at atemperature in the range of from 23 to 28° C.

Further, with respect to the atmosphere for the mixing, there is noparticular limitation. The mixing may be performed in general air or inan atmosphere of an inert gas, such as nitrogen gas.

<Fluoride Fluorescent Material>

The fluoride fluorescent material of the present embodiment comprisesfluoride particles represented by the general formula (I) above, and analkaline earth metal fluoride on the surface of the fluoride particles.

The fluoride fluorescent material of the present embodiment ispreferably produced by the method for producing a fluoride fluorescentmaterial of the present embodiment. The fluoride fluorescent materialcomprises the fluoride particles represented by the general formula (I)above as a fluorescent core, and a surface region in which an alkalineearth metal fluoride is present. In the present embodiment, an alkalineearth metal fluoride is present in the surface region, and therefore theconcentration of tetravalent manganese ions in the surface region havingan alkaline earth metal fluoride present therein is lower than theconcentration of tetravalent manganese ions in the fluoride particles.

The fluoride fluorescent material of the present embodiment hasexcellent water resistance, and is excited by a light having awavelength of a visible light on the short wavelength side to emit lightin the red range. Further, the fluoride fluorescent material exhibitingan emission spectrum having a narrow half band width is realized.

Hereinbelow, an example of the fluoride fluorescent material accordingto an embodiment of the present invention is described with reference tothe drawings. FIGS. 1 and 2 are individually drawings showing an exampleof the fluoride fluorescent material of the present embodiment and anenlarged view thereof. Fluoride fluorescent material 71 has fluorideparticle 73, and surface region 72 having an alkaline earth metalfluoride. Surface region 72 of the fluoride fluorescent material may beeither of a single layer in which, as shown in FIG. 1, the alkalineearth metal fluoride is uniformly present, or of a multilayer structurein which, as shown in FIG. 2, the alkaline earth metal fluorideconcentration is increased in the direction of from the inside ofsurface region 72 (i.e., the fluoride particle 73 side) to the outside.Further, the surface region may have a mode in which a plurality oflayers having a specific alkaline earth metal fluoride concentration arenot defined by definite interfaces in surface region 72 and the alkalineearth metal fluoride concentration is gradually increased in thedirection of from the inside to the outside of surface region 72.

In the present embodiment, from the viewpoint of achieving excellentemission luminance and water resistance, it is preferred that thealkaline earth metal fluoride comprises CaF₂. Further, in the presentembodiment, it is more preferred that the surface region is a layer ofthe alkaline earth metal fluoride.

In the fluoride fluorescent material obtained by the above-mentionedmethod, the whole of the particles are a fluoride fluorescent materialactivated by tetravalent manganese ions, which maintains properties of acolor reproduction range wider than that obtained using a conventionalfluoride fluorescent material, and the alkaline earth metal fluoridepresent on the surface of the fluoride fluorescent material can reducethe probability of manganese present on the surface. Therefore, evenwhen the surface of the fluoride fluorescent material particles isdissolved out in moisture, no or less tetravalent manganese ions arepresent in the surface region, and hence the formation of manganesedioxide derived from tetravalent manganese ions is suppressed. Thus, thesurface of the fluoride fluorescent material particles is prevented frombeing colored black, making it possible to suppress a lowering of thelight emission intensity.

In the present embodiment, the lower limit of the alkaline earth metalfluoride concentration of the fluoride fluorescent material is, forexample, 0.05% by weight or more, preferably 0.1% by weight or more,more preferably 0.2% by weight or more. The upper limit of theconcentration of the alkaline earth metal fluoride present in thesurface region is, for example, 5% by weight or less, preferably 3% byweight or less, more preferably 1% by weight or less. When the alkalineearth metal fluoride concentration is in the above range, thetetravalent manganese ion concentration can be reduced to approximatelyzero, improving the water resistance.

By virtue of having the above-mentioned construction of the fluoridefluorescent material, when the fluoride fluorescent material is incontact with water, a lowering of the emission luminance caused bycoloring due to the formation of manganese dioxide derived fromtetravalent manganese ions can be suppressed, realizing the fluoridefluorescent material having high water resistance.

The water resistance of the fluoride fluorescent material can beevaluated by, for example, an emission luminance maintaining ratio afterthe water resistance test, i.e., a ratio (%) of the emission luminanceafter the water resistance test to the emission luminance before thewater resistance test. The emission luminance maintaining ratio afterthe water resistance test is preferably 75% or more, more preferably 80%or more, further preferably 85% or more.

The water resistance test is conducted, specifically, by immersing thefluoride fluorescent material in water having a weight 1 to 5 times(preferably 3 times) the weight of the fluorescent material, andstirring the resultant water at 25° C. for one hour.

<Light Emitting Device>

The light emitting device of the present embodiment comprises a lightsource that generates light in the wavelength range of from 380 to 485nm, and the fluoride fluorescent material. The light emitting device mayfurther comprise another constituent member if necessary. The lightemitting device is not particularly limited, and can be appropriatelyselected from conventionally known light emitting devices. Examples ofthe light emitting devices include lighting apparatuses, such as afluorescent lamp, display apparatuses, such as a display and a radar,and a light source for a liquid crystal display apparatus.

By virtue of having the fluoride fluorescent material, the lightemitting device can achieve excellent long-term reliability.

(Light Source)

As a light source (hereinafter, also referred to as “excitation lightsource”), a light source that generates light in the wavelength range offrom 380 to 485 nm, which is the short wavelength region of a visiblelight, is used. The light source preferably has an emission peakwavelength in the wavelength range of from 420 to 485 nm, morepreferably in the wavelength range of from 440 to 480 nm. By using sucha light source, the fluoride fluorescent material can be efficientlyexcited, and thus a visible light can be effectively utilized. Further,by using an excitation light source having the above-mentionedwavelength range, a light emitting device having high light emissionintensity can be provided.

It is preferred that, as the excitation light source, a semiconductorlight emitting element (hereinafter, also referred to simply as “lightemitting element”) is used. By using a semiconductor light emittingelement as the excitation light source, there can be obtained a lightemitting device which has high efficiency and high linearity of theoutput with respect to the input and which is resistant to a mechanicalimpact and stable.

With respect to the light emitting element, one that generates lighthaving a wavelength of a visible light on the short wavelength side canbe used. For example, as a blue or green light emitting element, oneusing a nitride semiconductor (In_(X)Al_(Y)Ga_(1−X−Y)N, 0≤X, 0≤Y, X+Y≤1)can be used.

(Fluoride Fluorescent Material)

The details of the fluoride fluorescent material in the light emittingdevice are as mentioned above. The fluoride fluorescent material canconstitute the light emitting device by, for example, being contained inan encapsulation resin covering the excitation light source. In thelight emitting device having the excitation light source covered by anencapsulation resin containing the fluoride fluorescent material, partof the light emitted from the excitation light source is absorbed by thefluoride fluorescent material and emitted as a red light. By using theexcitation light source that generates light in the wavelength range offrom 380 to 485 nm, the emitted light can be more effectively utilized.Thus, a loss of the light emitted from the light emitting device can bereduced, so that the light emitting device having high efficiency can beprovided.

With respect to the amount of the fluoride fluorescent materialcontained in the light emitting device, there is no particularlimitation, and the amount of the fluoride fluorescent material can beappropriately selected according to, e.g., the excitation light source.

(Another Fluorescent Material)

It is further preferred that the light emitting device furthercomprises, in addition to the fluoride fluorescent material, anotherfluorescent material. Another fluorescent material may be anyfluorescent material which absorbs light from the light source andchanges the light in wavelength to light having a different wavelength.Another fluorescent material can constitute the light emitting devicelike the fluoride fluorescent material by, for example, being containedin an encapsulation resin.

Another fluorescent material is preferably at least one member selectedfrom the group consisting of, for example, nitride fluorescentmaterials, oxide nitride fluorescent materials, and sialon fluorescentmaterials, each activated mainly by a lanthanoid element, such as Eu orCe; alkaline earth halogen apatite fluorescent materials, alkaline earthmetal halogen borate fluorescent materials, alkaline earth metalaluminate fluorescent materials, alkaline earth silicates, alkalineearth sulfides, alkaline earth thiogallates, alkaline earth siliconnitrides, and germanates, each activated mainly by a lanthanoid element,such as Eu, or a transition metal element, such as Mn; rare earthaluminates and rare earth silicates, each activated mainly by alanthanoid element, such as Ce; and inorganic and organic complexes eachactivated mainly by a lanthanoid element, such as Eu.

Specific examples of other fluorescent materials include (Ca, Sr,Ba)₂SiO₄:Eu, (Y, Gd)₃(Ga, Al)₅O₁₂:Ce, (Si, Al)₆(O, N)₈:Eu(β-sialon),SrGa₂S₄:Eu, (Ca, Sr)₂Si₅N₈:Eu, CaAlSiN₃:Eu, (Ca, Sr)AlSiN₃:Eu,Lu₃Al₅O₁₂:Ce, and (Ca, Sr, Ba, Zn)₈MgSi₄O₁₆(F, Cl, Br, I):Eu.

By using another fluorescent material in the light emitting device, thelight emitting device having various color tones can be provided.

When the light emitting device further comprises another fluorescentmaterial, the amount of the fluorescent material contained is notparticularly limited, and may be appropriately selected so as to obtaindesired light emission properties.

When the light emitting device further comprises another fluorescentmaterial, the light emitting device preferably comprises a greenfluorescent material, more preferably a green fluorescent material thatemits light in the wavelength range of from 495 to 573 nm when itabsorbs light in the wavelength range of from 380 to 485 nm. The lightemitting device comprising a green fluorescent material can be morepreferably applied to a liquid crystal display apparatus.

With respect to the form of the light emitting device, there is noparticular limitation, and the form can be appropriately selected fromforms generally used. As examples of forms of the light emitting device,there can be mentioned a shell type and a surface mount type. Generally,the shell type indicates a light emitting device having a resin whichconstitutes the outer surface and which is formed into a shell shape.The surface mount type indicates a light emitting device having acontainer portion in a depressed form filled with a light emittingelement as a light source and a resin. As a further example of the formof the light emitting device, there can be mentioned a light emittingdevice in which a light emitting element as a light source is mounted ona printed circuit board in a flat plate form, and an encapsulation resincontaining the fluoride fluorescent material is formed in a lens shapeso as to cover the light emitting element.

Hereinbelow, an example of the light emitting device according to thepresent embodiment will be described with reference to the drawings.FIG. 3 is a diagrammatic cross-sectional view showing an example of thelight emitting device of the present embodiment. FIG. 4 is adiagrammatic plan view showing an example of the light emitting deviceof the present embodiment. This light emitting device is an example of asurface mount-type light emitting device.

Light emitting device 100 has light emitting element 10 comprised of agallium nitride compound semiconductor that generates light having awavelength of a visible light on the short wavelength side (for example,380 to 485 nm), and molded piece 40 which has placed thereon lightemitting element 10. Molded piece 40 has first lead 20 and second lead30, and they are integrally molded using a thermoplastic resin orthermosetting resin. Molded piece 40 has formed a depressed portionhaving a bottom and sides, and has light emitting element 10 placed onthe bottom of the depressed portion. Light emitting element 10 has apair of positive and negative electrodes, and a pair of the positive andnegative electrodes is electrically connected to first lead 20 andsecond lead 30 through wire 60. Light emitting element 10 isencapsulated by encapsulation member 50. As encapsulation member 50, athermosetting resin, such as an epoxy resin, a silicone resin, anepoxy-modified silicone resin, or a modified silicone resin, ispreferably used. Encapsulation member 50 contains fluoride fluorescentmaterial 70 which changes the wavelength of light from light emittingelement 10.

<Image Display Apparatus>

The image display apparatus comprises the at least one light emittingdevice. With respect to the image display apparatus, there is noparticular limitation as long as the apparatus comprises the lightemitting device, and the apparatus can be appropriately selected fromconventionally known image display apparatuses. The image displayapparatus has, for example, in addition to the light emitting device, acolor filter member and a light transmission control member.

By virtue of having the light emitting device, the image displayapparatus exhibits excellent luminance and color reproduction range aswell as excellent long-term reliability.

EXAMPLES

Hereinbelow, the present invention will be described in more detail withreference to the following Examples, which should not be construed aslimiting the scope of the present invention.

Comparative Example 1

The method for preparing fluoride particles in Comparative Example 1constituting a fluorescent core is first described below. 16.25 g ofK₂MnF₆ was weighed and dissolved in 1,000 g of a 55% by weight aqueousHF solution to prepare solution A, so that the charged compositionbecame as shown in Table 1. Separately, 195.10 g of KHF₂ was weighed anddissolved in 200 g of a 55% by weight aqueous HF solution to preparesolution B. Further, 450 g of a 40% by weight aqueous H₂SiF₆ solutionwas weighed and prepared as solution C. While stirring solution A,solution B and solution C were simultaneously added to solution A, andthe resultant deposits were separated, and then washed with IPA(isopropyl alcohol), and dried at 70° C. for 10 hours to preparefluoride particles in Comparative Example 1 constituting a fluorescentcore.

Example 1

1.18 g of calcium nitrate tetrahydrate was weighed and dissolved in DIW(deionized water) to prepare 200 g of an aqueous solution containingcalcium ions in an amount of 0.1% by weight, so that the conditions forcharged reaction solution became as shown in Table 2. Then, 6.9 g of a30% aqueous H₂O₂ solution was added as a reducing agent to the preparedaqueous solution, and then 20 g of the fluoride particles prepared inComparative Example 1 were added thereto, and the resultant mixture wasstirred at 25° C. for one hour to effect a reaction, forming calciumfluoride on the surface of the fluoride particles. The resultantdeposits were separated, and then washed with IPA, and dried at 70° C.for 10 hours to prepare a fluoride fluorescent material in Example 1.

Examples 2 to 4

Fluoride fluorescent materials in Examples 2 to 4 were individuallyprepared by substantially the same method as the method for the fluoridefluorescent material in Example 1 except that the conditions for chargedreaction solution, the amount of the fluoride particles used, and thereaction conditions were changed to those shown in Table 2.

Example 5

2.11 g of magnesium nitrate hexahydrate was weighed and dissolved in DIWto prepare 200 g of an aqueous solution containing magnesium ions in anamount of 0.1% by weight, so that the conditions for charged reactionsolution became as shown in Table 2. Then, 6.9 g of a 30% aqueous H₂O₂solution was added as a reducing agent to the prepared aqueous solution,and then 20 g of the fluoride particles prepared in Comparative Example1 was added thereto, and the resultant mixture was stirred at 25° C. forone hour to effect a reaction, forming magnesium fluoride on the surfaceof the fluoride particles. The resultant deposits were separated, andthen washed with IPA, and dried at 70° C. for 10 hours to prepare afluoride fluorescent material in Example 5.

Example 6

A fluoride fluorescent material in Example 6 was prepared bysubstantially the same method as in Example 5 except that the conditionsfor charged reaction solution, the amount of the fluoride particlesused, and the reaction conditions were changed to those shown in Table2.

TABLE 1 Solution A Solution C Charged Solution B Charged amount (g)Charged amount (g) amount (g) 55% 55% 40% Charged composition AqueousAqueous Aqueous (mol) HF HF H₂SiF₆ K Si Mn F K₂MnF₆ solution KHF₂solution solution Comparative 2 0.95 0.05 6 16.25 1000 195.10 200 450example 1

TABLE 2 Conditions for charged reaction solution Fluoride particlesReducing Fluorescent Reaction conditions Alkaline Ion agent material inReaction earth metal concentration concentration Comparative solutionReaction ion type (wt %) (wt %) Example 1 (g) amount (g) time (hr)Example 1 Ca²⁺ 0.1 1 20 200 1 Example 2 Ca²⁺ 0.1 1 20 400 1 Example 3Ca²⁺ 0.1 1 20 200 4 Example 4 Ca²⁺ 0.5 1 20 200 1 Example 5 Mg²⁺ 0.1 120 200 1 Example 6 Mg²⁺ 0.5 1 20 200 1

(Evaluation of Emission Luminance Characteristics)

With respect to the above-obtained fluoride fluorescent materials inExamples 1 to 6 and Comparative Example 1, prior to a water resistancetest, general measurement of the emission luminance characteristics wasconducted. The relative emission luminances determined when the emissionluminance in Comparative Example 1 measured before a water resistancetest is taken as 100.0% are as shown in Table 3 below. The emissionluminance characteristics were measured under conditions at anexcitation wavelength of 460 nm for a reflective luminance.

(Water Resistance Test)

With respect to the above-obtained fluoride fluorescent materials inExamples 1 to 6 and Comparative Example 1, a water resistance wasevaluated. A water resistance test was conducted in which 5 g of thefluorescent material was immersed in 15 g of pure water and stirred at25° C. for one hour and then, the resultant material was separated andwashed with IPA, and dried at 70° C. for 10 hours. Thus, the fluoridefluorescent materials after the water resistance test were individuallyobtained.

With respect to the fluoride fluorescent materials after the waterresistance test, measurement of the emission luminance characteristicswas performed in the same manner as mentioned above. The results areshown in Table 3. A ratio of the relative luminance after the waterresistance test to the relative luminance before the water resistancetest is also shown as a luminance maintaining ratio (%).

TABLE 3 Emission luminance characteristics Emission luminancecharacteristics after water resistance test Chromaticity RelativeChromaticity Relative Luminance coordinates luminance coordinatesluminance maintaining x y (%) x y (%) ratio (%) Comparative 0.677 0.313100.0 0.677 0.313 72.4 72.4 example 1 Example 1 0.677 0.312 104.7 0.6770.313 96.8 92.5 Example 2 0.677 0.313 104.7 0.677 0.312 95.1 90.8Example 3 0.677 0.313 105.7 0.677 0.312 98.0 92.8 Example 4 0.677 0.312105.9 0.677 0.312 96.8 91.4 Example 5 0.679 0.312 103.0 0.678 0.312 81.378.9 Example 6 0.679 0.312 103.9 0.678 0.312 82.5 79.4

As apparent from Table 3, in Examples 1 to 6, the luminance is improved.The reason for this is presumed that an alkaline earth metal fluoridehaving a refractive index different from that of the fluorescentmaterial was formed on the surface of the fluorescent materialparticles, so that the light extraction efficiency was improved.

As also apparent from Table 3, the fluoride fluorescent material havingno alkaline earth metal fluoride in Comparative Example 1 had anemission luminance lowered to 72.4%. By contrast, the fluoridefluorescent materials in all the Examples achieved a luminancemaintaining ratio of 75% or more. Especially, the fluoride fluorescentmaterials in Examples 1 to 4, in which the alkaline earth metal ions arecalcium ions, achieved a luminance maintaining ratio of 90% or more,which has confirmed the utility of the present Examples.

INDUSTRIAL APPLICABILITY

The fluoride fluorescent material of the present invention and the lightemitting device using the same can be used in, for example, afluorescent character display tube, a display, a PDP, a CRT, an FL, anFED, and a projector tube, particularly, in a backlight light sourceusing a blue light emitting diode as a light source and having extremelyexcellent light emission properties, an LED display, a light source forwhite lighting, a signal, a light switch, and various types of sensorsand indicators, and especially in the use of display, they exhibitexcellent light emission properties.

The foregoing description of the exemplary embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in the art. Theexemplary embodiments were chosen and described in order to best explainthe principles of the invention and its practical applications, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with the various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

All publications, patent applications, and technical standards mentionedin this specification are herein incorporated by reference to the sameextent as if each individual publication, patent application, ortechnical standard was specifically and individually indicated to beincorporated by reference.

The invention claimed is:
 1. A fluoride fluorescent material comprising:fluoride particles represented by the following general formula (I):K₂[M_(1−a)Mn⁴⁺ _(a)F₆]  (I) wherein M is at least one element selectedfrom the group consisting of elements belonging to Groups 4 and 14 ofthe Periodic Table, and a satisfies the relationship: 0<a<0.2; and analkaline earth metal fluoride on the surface of the fluoride particles,wherein the surface of the fluoride particles has a multilayer structureincluding at least a first layer adjacent to the fluoride particle and asecond layer adjacent to the first layer, wherein the concentration ofthe alkaline earth metal fluoride increases within the multilayerstructure in a direction away from the fluoride particle, and wherein anemission luminance maintaining ratio, after a water resistance test, is90% or more.
 2. The fluoride fluorescent material according to claim 1,wherein the alkaline earth metal fluoride comprises CaF₂.
 3. A lightemitting device comprising: a light source that generates light at 380to 485 nm; and the fluoride fluorescent material according to claim 1.4. The light emitting device according to claim 3, further comprising agreen fluorescent material that emits light at 495 to 573 nm when itabsorbs light at 380 to 485 nm.
 5. A light source for a liquid crystaldisplay apparatus, the light source comprising the light emitting deviceaccording to claim
 3. 6. An image display apparatus comprising the lightemitting device according to claim
 3. 7. The fluoride fluorescentmaterial according to claim 1, wherein the concentration of the alkalineearth metal fluoride gradually increases in a direction away from thefluoride particle.
 8. The fluoride fluorescent material according toclaim 1, wherein the concentration of the alkaline earth metal fluoridein the first layer is less than the concentration of the alkaline earthmetal fluoride in the second layer.
 9. The fluoride fluorescent materialaccording to claim 1, wherein the multilayer structure lacks a definiteinterface between the first and second layers.